Expansion of the Universe

Possible scenarios for the expansion (and possibly contraction) of the universe: the bottom orange curve represents a closed, high density universe which
expands for several billion years, then ultimately turns around and collapses under its own
weight. The green curve represents a flat, critical density universe
in which the expansion rate continually slows down (the curves becomes ever more horizontal).
The blue curve shows an open, low density universe whose expansion
is also slowing down, but not as much as the previous two because the pull of gravity is not as
strong. The top (red) curve shows a universe in which a large fraction
of the matter is in a form dubbed "dark energy" which is causing the expansion of the
universe to speed up (accelerate). There is growing evidence that our universe is following the
red curve.